1. Field of the Invention
The present invention relates to a degradation judgment device for a secondary battery.
2. Description of the Related Art
In recent years, a secondary battery has been available for a wide range of applications such as a backup power supply that is used, e.g., when a commercial power supply comes to a stop, a power supply for portable equipment, or a power supply for power-driven equipment.
When a secondary battery is charged/discharged repeatedly, or when a secondary battery is connected to main equipment and is charged repeatedly by the amount corresponding to self-discharge while no load is applied to the battery itself, the secondary battery is degraded and cannot perform a desired function. Then, the degraded secondary battery should be replaced with a new secondary battery.
As a technique for judging the degradation of the secondary battery, a method for judging the life of the secondary battery with a voltage during trickle charge has been proposed, e.g., by JP 8(1996)-293329 A.
However, there are some problems in this conventional configuration.
To perform the above degradation judgment, the charging current is required not only to be a constant current, but also to be maintained constant with high accuracy because this is useful in suppressing voltage fluctuations caused by a change in charging current. For example, an A-size nickel-metal hydride battery has an internal resistance (IR) of about 20 mΩ. When the charging current is 1A, a current detection accuracy of several tens of mV is necessary for the degradation judgment. Accordingly, a charging circuit must be expensive.
In the conventional degradation judgment, the charging current should be a constant current. Therefore, in some cases, the degradation judgment cannot be applied to batteries other than an alkaline storage battery.
Moreover, the conventional degradation judgment circuit cannot judge degradation of a battery unless the battery is being charged.